generic/317: use id to get user id

Greping username "fsgqa" in passwd file to get user id doesn't
always work, because the given username could match multiple
entries, e.g. when we have user "123456-fsgqa" (used by other tests)
in passwd file.

Fix it by using "id -u $qa_user" to get the user id directly.

Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/generic/317

@@ -61,7 +61,7 @@ _require_scratch
 _require_user
 _require_ugid_map
 _require_userns
-qa_user_id=`grep $qa_user /etc/passwd |awk -F: '{print $3}'`
+qa_user_id=`id -u $qa_user`
 
 _filter_output()
 {